I suppose it's a good thing that "it doesn't have to be a good thing for us"....

Cairo (CNN) -- Angry protesters climbed the walls of the U.S. Embassy in Cairo on Tuesday and hauled down the American flag, replacing it with a black standard with Islamic emblems, apparently in protest of the production of a film thought to insult the Prophet Mohammed.

The incident prompted a volley of warning shots to be fired as a large crowd gathered outside, said CNN producer Mohammed Fahmy, who was on the scene.

The replacement flag read, "There is no God but Allah and Mohammad is his messenger."

Others expressed more general grievances about U.S. policy, chanting anti-American slogans and holding up bits of a shredded American flag to television camera crews in front of the embassy.

An embassy operator told CNN that the facility had been cleared of diplomatic personnel earlier Tuesday, ahead of the apparent threat, while Egyptian riot police were called to help secure the area.

The U.S. Embassy said in a statement Tuesday that it "condemns the continuing efforts by misguided individuals to hurt the religious feelings of Muslims -- as we condemn efforts to offend believers of all religions."

"Respect for religious beliefs is a cornerstone of American democracy," the statement said. "We firmly reject the actions by those who abuse the universal right of free speech to hurt the religious beliefs of others."

The incident occurred on the 11th anniversary of the September 11 attacks in New York, Washington and Pennsylvania, as crowds gathered in somber remembrance of the nearly 3,000 people killed that day.

It is not clear which film upset the protesters in Cairo.

In 2008, an anti-Muslim and anti-immigration parliamentarian from The Netherlands sparked international outcry when he produced a film that portrayed Islam as a violent religion.

Geert Wilders' film "Fitna," which he released online, featured images of terrorist acts superimposed over verses from the Quran.